German Published Patent Application No. 195 37 814 A1 describes a known method for fabricating a micromechanical acceleration sensor.
Although the present invention as well as the problem definition underlying the present invention are applicable in principle to any micromechanical components, they will be elucidated with reference to this known surface-micromechanical acceleration sensor.
FIG. 2 shows a schematic, cross-sectional representation of the known micromechanical acceleration sensor, fabricated using known methods. FIG. 3 is a schematic plan view of the subject matter of FIG. 2. FIG. 4 is an enlarged schematic representation of the contact region of the known micromechanical acceleration sensor according to FIG. 2 to elucidate the problem definition underlying the present invention.
In FIGS. 2 through 4, reference symbol 100 generally denotes a micromechanical acceleration sensor. The micromechanical sensor includes a substrate 10, a bottom oxide 1, and a top oxide 2. Sensor 100 also includes a printed circuit trace of LPCVD polysilicon 3 buried between the two oxides 1, 2, and top oxide 2 includes contact holes 4. A layer of epitaxial polysilicon 6 (i.e., polysilicon deposited in an epitaxial reactor to achieve a higher deposition rate) is also provided in sensor 100. Further included in sensor 100 are a bonding pad of aluminum 7, a solder glass layer 8, first trenches 9, second trenches 9', and a bonding pad base (socket) 20 of epitaxial polysilicon, also referred to as a contact element. Other elements arranged as elements of the sensor 100 include a frame structure of epitaxial polysilicon 21, a movable element 25 having an anchored region 22 and a free-standing region 23, a polysilicon contact plug 60 as a part of layer 6 of polysilicon, an Si protective cap or wafer cap 13, contact regions B1-B5, circuit trace regions L1-L5, a sensor core region I, a capping (encapsulation) edge area II, a bonding pad region III. Reference character S identifies a dirt particle.
When the known technique is used to fabricate this acceleration sensor, the thicknesses of the various layers are typically as follows:
aluminum bonding pad 7: 1.35 .mu.m layer 6 of polysilicon: 10.30 .mu.m second oxide 2: 1.60 .mu.m buried circuit trace layer 3: 0.45 .mu.m first oxide 1: 2.50 .mu.m
When the customary technique is performed, patterns are exposed in the 10 .mu.m thick layer 6 of polysilicon by forming trenches and by removing the underlying sacrificial layer (oxide 1, 2).
To obtain freely movable sensor elements in region I, the undercut-type etching is not only necessary, but also desired. On the other hand, in region III, an undercut-type etching is not at all desired, and would be detrimental for reasons elucidated in the following. Region II is completely covered with polysilicon and is used to hermetically encapsulate the sensor 100 with the aid of the Si protective cap 13.
As is apparent from the enlarged representation of region III in FIG. 4, the 10 .mu.m thick bonding pad base 20 of polysilicon, which has trenches formed in it in the same process step in which the movable structure is formed in sensor core region I, bears aluminum bonding pad 7 and is connected via contacting plug 60 to the underlying, thin LPCVD polysilicon circuit trace 3. Prior to the etching of the sacrificial layer, LPCVD polysilicon circuit trace 3 is embedded between the two oxide layers 1, 2.
When each of the first and second oxide layer 1, 2 is etched in that region which is simultaneously etched as a sacrificial layer for movable element 25 of the sensor 100, top oxide 2 is completely removed, and lower oxide 1 is partially removed. The result is the undercut-type etching of bonding pad base 20 and of circuit trace layer 3, as described.
At these locations, conductive dirt particles S, produced, in particular, during sawing (slicing) operations as sludge from the saw, can accumulate and lead to electrical shunting between bonding pad base 20 and substrate 10, or between bonding pad base 20 and circuit trace layer 3.
In addition, breaks can occur in the buried circuit trace layer 3 or in the undercut (laterally etched) epitaxial polysilicon edges of bonding pad base 20, particularly during the high-pressure scrubbing performed subsequently to the sawing operation. The consequences are possible shunts, an increased resistance, and dirty break edges.
Generally it is a drawback that, subsequent to the etching of the sacrificial layer, circuit trace layer 3 is no longer covered with a dielectric material; this is in violation of standard IC- design safety regulations.
Generally, therefore, it would be advantageous to implement a method that easily avoids shunts of this kind.
A customary approach used to avoid undercutting the bonding pads provides for covering second trenches 9' around the bonding pads with a negative resist.
In terms of process technique, the disadvantage of this approach is the fact that difficulties arise when working with step heights of more than about 10 .mu.m, and the fact that this approach is not even practicable for all sensor structures. Also, the negative resist must be removed immediately following the HF (high frequency) gas-phase etching of the sacrificial layer, since the HF causes it to become saturated, so that it can no longer prevent the undercut-type etching. Completely removing the resist in the trenches turns out to be complicated in this context, and even small amounts of residue can cause the comb-type structures to become cemented.
A second approach provides, at least temporarily, an additional protective layer in a designated region underneath and around the bonding pad base on the sacrificial layer to prevent the sacrificial layer from being completely undercut underneath the bonding pad base. This makes the manufacturing process more costly.